X-ray imaging has been used in the medical field and for radiology in general, such as non-destructive testing and x-ray computed tomography. Conventional radiography systems use x-ray absorption to distinguish differences between different materials, such as normal and abnormal human tissues.
Conventional x-ray radiography measures the projected x-ray attenuation, or absorption, of an object. Attenuation differences within the object provide contrast of embedded features that can be displayed as an image. For example, cancerous tissues generally appear in conventional radiography because these tissues are more dense than the surrounding non-cancerous tissues. The best absorption contrast is generally obtained at x-ray energies where the absorption is high. Conventional radiography is typically performed using lower x-ray energy in higher doses to allow greater absorption and, thus, better contrast and images. Using x-rays having higher energy generally requires a lower dosage to be used because of patient safety concerns. In general, as the x-ray energy level increases and the x-ray dose used decreases, the quality of the conventional radiography image lessens.
Diffraction Enhanced Imaging (DEI), for example, as disclosed in U.S. Pat. No. 5,987,095, issued to Chapman et al., and U.S. Pat. No. 6,577,708, issued to Chapman et al., is a radiographic technique that derives contrast from an object's x-ray absorption, refraction and ultra-small-angle scattering properties. DEI can be used to detect, analyze, combine and visualize the refraction, absorption and scattering effects upon an image of an object. DEI is particularly useful for relatively thick and thus highly absorbing materials. Compared to the absorption contrast of conventional radiography, the additional contrast mechanisms, refraction and scatter, of DEI allow visualization of more features of the object.
DEI, and the method of this invention, can use highly collimated x-rays prepared by x-ray diffraction from perfect single-crystal silicon. These collimated x-rays are of single x-ray energy, practically monochromatic, and are used as the beam to image an object. A schematic of a DEI setup is shown in FIG. 1. In this case, the collimated x-rays are prepared by two silicon (3, 3, 3) crystals of a monochromator 11. Once this beam passes through the object, another crystal of the same orientation and using the same reflection is introduced. This crystal is called an analyzer 30. If this crystal is rotated about an axis perpendicular to the plane shown in FIG. 1, the crystal will rotate through a Bragg condition for diffraction and the diffracted intensity will trace out a profile that is called a rocking curve, such as shown in FIG. 2. The profile will be roughly triangular and will have peak intensity close to that of the beam intensity striking the analyzer crystal. The width of the profile is typically a few microradians wide, for example 3.6 microradians within a full width of half maximum (FWHM) at 18 keV using the silicon (3, 3, 3) reflection. The character of the images obtained change depending on the setting of the analyzer crystal. To extract refraction information, the analyzer is typically set to the half intensity points on low (RL) and high (RH) angle sides of the rocking curve. At least two images are obtained by a detector at different angled positions, for example, one at each of the low and high angle sides of the rocking curve, of the crystal analyzer. The images are mathematically combined to obtain images, such as a refraction angle image.
DEI can use higher x-ray energy levels than generally available for obtaining absorption images using conventional radiography. One advantage of using higher x-ray energies is a lower dose is required. A second advantage is that higher x-ray energies reduce the flux required of an imaging system, as the transmission through the object is high. However, applying DEI at higher x-ray energy levels with a lower dose can lead to the loss of a good absorption-based radiograph, as x-ray absorption is lower at higher energy levels. Therefore, acceptance of DEI by radiologists may be limited without the ability to create an image resembling a conventional radiograph.
There is a need for an imaging method that provides an image having characteristics similar to a conventional radiograph at higher x-ray energy levels and lower doses, such as used in DEI. There is a need for an imaging method that provides an image having characteristics similar to a conventional radiograph, and where the image is a property of the object with no direct dependence on the imaging x-ray energy level.